The present invention relates to radio receiver used for mobile radio communications, or more in particular to a radio receiver such as a digital radio receiver or an FM receiver with delayed detection type using a limiter amplifier.
FIG. 1 is a block diagram showing a receiving section of a conventional digital radio transceiver. A signal input from an antenna 1 is amplified by a low-noise amplifier 2, and after the unrequited signal components are removed by a bandpass filter 3, the resultant signals are frequency converted into a first intermediate frequency band by a first mixer 4. Only the required signal is selected by a bandpass filter 5, and after being frequency-converted into a second intermediate frequency by a second mixer 7, is waveform-shaped by a roll-off filter 8. The resulting signal provided from the filter 8 is amplified by a second intermediate frequency amplifier 9 and demodulated by a demodulator 10.
Now, the problem points of the receiver shown in FIG. 1 will be explained in detail. When the signal level received by the antenna 1 increases, the signal in the second intermediate frequency amplifier 9 including several stages of limiter amplifiers begins to be limited in amplitude by a last limiter amplifier stage. With the increase in the receiving level, the signal in the second intermediate frequency amplifier 9 is limited in amplitude by each limiter amplifier stage progressively toward the preceding stages from the last stage. A receiving level where all the limiter amplifiers making up the second intermediate frequency amplifier 9 limit the signal in amplitude is assumed to be P.sub.0, and a receiving level where the output of the second mixer 7 is saturated is assumed to be P.sub.1. Generally, level P.sub.1 is set larger than P.sub.0. Therefore, when the receiving level reaches P.sub.1, the second mixer 7 preceeding the roll-off filter 8 begins to be saturated, then a transmission distortion occurs thereby deteriorating a bit error rate. FIG. 3 shows an example of bit error rate characteristic against the receiving level of the receiver of FIG. 1. Also, FIG. 4 shows an example of the output characteristics of the second mixer 7 with respect to the receiving level. In the case where the receiving level is more than level P.sub.1, as shown in FIG. 4, the output signal of the second mixer 7 beings to be distorted by saturation, and the bit error rate is rapidly deteriorated as shown in FIG. 3. This phenomenon frequently occurs when the transmitter and the receiver are closely located to each other as when portable radio transceivers are in proximity to each other or to a base station, and deteriorates the transmission quality of radio communications.
A method known to obviate the above-mentioned problem is to increase the current consumption of the second mixer 7 or to increase the power supply voltage. The second mixer 7 and the second intermediate frequency amplifier 9, however, are generally supplied as integrated circuits such as seen in Phillips' SA626 or Motorola's MC13156 available on the market. The use of these integrated circuits cannot change the DC bias of the second mixer 7.
Several other methods for solving these problems are publicly known. A representative case is disclosed in JP-A-61-222326 entitled "Receiver". This publication concerns a system comprising a variable attenuator inserted between the bandpass filter 5 and the second mixer 7 of the receiver shown in FIG. 1 and the attenuation amount of the variable attenuator is controlled in accordance with the receiving level. FIG. 2 is a block diagram showing the configuration of a receiver according to this publication. In FIG. 2, the component parts having the same functions as the receiver shown in FIG. 1 are denoted by the same reference numerals respectively. The difference between the system of FIG. 1 and the prior art will be described below.
In the published system, a variable attenuator 11 is interposed between the bandpass filter and the second mixer 7. The output of the bandpass filter 8 is applied to a second intermediate frequency amplifier 9 on the one hand, while a part of the signal is amplified by an amplifier 12 and applied to a detector 13 to detect the receiving level. The variable attenuator 11 is controlled in such a manner as to reduce the attenuation amount when the receiving level is low and to increase the attenuation amount when the receiving level is high, in accordance with the receiving level detected by the detector 13. Another part of the output of the bandpass filter 8 is applied to an interference detector 14 for interference detection. As described above, the signal distortion occured in the mixer 7 can be lowered up to a high receiving level by controlling the variable attenuator 11.
A variable gain amplifier may be used in place of the variable attenuator 11 as a means for solving the problem, similar to that presented by FIG. 2.
As described above, a receiver as shown in FIG. 1 poses the problem of the bit error rate deterioration due to the distortion of the second mixer 7 when the receiving level is excessively high. The problem is especially conspicuous when integrated circuits available on the market are used. Also, in the case where a receiver as shown in FIG. 2 is used for mobile radio communications, an abrupt change in electric field intensity due to fading or the like poses the problem of the attenuator control being unable to follow the change. Further, the amplifier 12 and the detector 13 are required to drive the variable attenuator 11, thereby leading to the problem of complicating the circuit configuration.